Firmly adhering color and/or effect coatings on metal strips, or coils, particularly those consisting of the conventional utility metals, such as zinc, aluminum or bright, galvanized, electrolytically zincked, and phosphated steel, are produced preferably by means of the coil coating process (Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag, Stuttgart, N.Y., 1998, page 617, “roll coating”, and page 55, “coil coating”; A. Goldschmidt and H.-J. Streitberger, BASF-Handbuch Lackiertechnik, Vincentz Verlag, Hanover, 2002, “4.2.1.2 brushing, rolling, roller, flow, and casting methods (coating materials directly to the object)”, pages 521 to 527, especially 523, and “7.4 Coil Coating”, pages 751 to 756).
Where highly pigmented topcoats which are matt and/or of only slight gloss are to be produced, it is advisable for that purpose to use coating materials which can be cured with actinic radiation and which can preferably be cured rapidly by means of electron beams (EBC) (cf., e.g., B. A. Goldschmidt and H.-J. Streitberger, BASF-Handbuch Lackiertechnik, Vincentz Verlag, Hanover, 2002, pages 638 to 641). On account of the high pigment content, curing with UV radiation is difficult if not impossible.
Coil coating processes for producing color and/or effect coatings, where first a primer film is applied to the coils and is partly or fully cured, and then a topcoat film is applied and, alone or together with the primer film, is cured fully with actinic radiation, are known.
Here and below, actinic radiation means electromagnetic radiation, such as near infrared (NIR), visible light, UV radiation, X-rays, and gamma radiation, especially UV radiation, and also corpuscular radiation, such as electron beams, beta radiation, alpha radiation, proton beams, and neutron beams, especially electron beams.
For example, in the case of the process known from U.S. Pat. No. 3,959,521, the first film applied to the coils is a primer film comprising olefinically unsaturated double bonds and free isocyanate groups. The primer film can be cured with electron beams or UV radiation to give the primer coat. Applied atop the primer coat is a substantially solvent-free, pigmented topcoat material based on polyesters which contain two acrylate groups. The resulting topcoat film is cured with electron beams to give the topcoat.
In the similar process known from German patent application DE 44 21 558 A1 the pigmented topcoat material comprises prepolymers containing acrylate groups, such as polyurethane acrylates, polyester acrylates, polyether acrylates, epoxy acrylates or full acrylates, or mixtures thereof, which may have been blended with low-viscosity reactive diluents. The topcoat material may further comprise unspecified organic, inorganic or metallic fillers, matting agents, rheological additives, waxes, flow control agents, lubricants, devolatilizers, defoamers, adhesion promoters, dispersing assistants, wetting agents, antisettling agents, antistats, stabilizers, light stabilizers, corrosion inhibitors, and other substances known to the skilled worker, individually or in a mixture.
In the case of the similar process known from International patent application WO 01/32321 A2 a topcoat material curable with UV radiation or electron beams is applied to a thermally curable basecoat film or thermally cured basecoat. The topcoat material comprises oligomers or polymers which contain acrylate, styrene, vinyl and/or allyl groups, preferably acrylate groups. Preference is given to using polyacrylates, urethane acrylates, epoxy acrylates, polyester acrylates and/or amino acrylates. The topcoat material may be solvent-free or may include solvent. It may further comprise reactive diluents (monomers), such as isobornyl acrylate, etc., and adhesion promoters.
The known processes yield color and/or effect coatings which are firmly adhering, hard, flexible, and deformable, and which are said to afford effective corrosion control.
In terms of energy consumption and their belt speed, however, it is necessary to further improve the known coil coating processes, in order to satisfy the continually growing requirements of the market, particularly of coil coaters.
The coating materials that are employed in this field must likewise be continually further improved in terms of their preparation, storage stability, and handling, including their application.
Similarly, the known, color and/or effect coatings produced with these materials must likewise be further improved in their hardness, flexibility, deformability, substrate adhesion, intercoat adhesion, and corrosion control effect, in order to satisfy the likewise continually growing demands of the coil coaters' industrial customers.